ACCEPTANCE TESTING
5-1-34
Accuracy of DDM (Difference Depth of Modulation) waveform
SPECIFICATION
±
0.03% of depth
±
0.02 of setting
This specification is checked by measuring the modulation channel balance at 19.9% and
40% SDM (with and without final divide-by-5 internal attenuator).
TEST EQUIPMENT
Description Minimum
specification
Example
Spectrum analyzer
DC to 25 kHz
Aeroflex 2392
or
3 Hz filter
FFT analyzer
(1)
Connect the test equipment as shown in Fig. 5-1-23.
(2)
Set up the test equipment as follows:-
Unit under test
ILS mode
Carrier freq
108.1 MHz
LF output
Monitor AM drive
SDM 40%
DDM
0%
Dominant tone
90 Hz
Spectrum analyzer
Span/div 20
Hz
Frequency
0 to 200 Hz
Ref level
20 dBm
Resolution bandwidth
3 Hz
Input DC
coupled
The spectrum analyzer should display equal amplitude tones of 90 Hz and 150 Hz.
(3)
Reset the test equipment as follows:
Unit under test
DDM 40%
The 150 Hz tone should now be suppressed. Check that the residual level is at least
52 dB lower than the 90 Hz tone.
(4)
Reset the test equipment as follows:
Unit under test
SDM 19.9%
DDM 19.9%
The 150 Hz tone should still be suppressed. Check that the residual level is at least 52 dB
lower then the 90 Hz tone.
